I might have the solution. Go to http://www.facebook.com/developers/ and click on your Application, at the right of the page. Once you're on the new page, click on "Edit Settings".

On the "Website" tab, paste the link to your forum on Site URL. Then hit save, go to your website, see if it works.

I remember there used to be a "Connect" tab a few months ago, but the Devs seem to have updated it.

* Not meaning to spam, but I own a forum called Facebook Forum where we provide Facebook help and support to over 50 people a day, over 5,000 solved problems already. [ STOP! POSTING PROMOTIONAL SPAM LINKS ] in case you still or ever need help with Facebook